The present invention relates to a method able rapidly to detect the initiating of short-circuit conditions in an electrical network, as well as to a device which uses this method.
The invention will be illustrated with particular reference to a preferred application in a low-voltage electrical power distribution network, although this is not to be understood as limiting, and in any event the invention applies in general to both medium and low-voltage networks.
In modern automatic circuit breakers for low and medium voltage, devices for detecting faults are integrated in the circuit breakers, and such detection of faults is often performed by means of devices of electronic type.
However, the detection of short-circuits, characterized by very high current and by the need for very rapid actuation so as to eliminate the fault, is still carried out at the present time by electromechanical devices based on the electrodynamic effect.
The calibration of these short-circuit detection devices is very difficult and is carried out on an empirical basis. This leads to low accuracy in the determination of the actuation times of the circuit breaker.
Moreover, with these electromechanical devices it is not possible to gather, in the case of a short-circuit fault, detailed information about the fault such as, for example, the peak current characterizing the fault, the complex impedance of the circuit or the angle of the voltage at the moment of extinction of the fault. This information would be useful among other things for diagnostic and statistical purposes.
Finally, in an automatic circuit breaker based on the known short-circuit detection mechanism, it is not possible to prevent the opening of the circuit breaker once the fault has been detectd. However, in many situations, such as for example when the circuit breaker is part of a power distribution network which comprises other circuit breakers, it is desirable for just one of them to open the circuit, even if the fault was detected by several circuit breakers simultaneously.
There is therefore a technical need to make available a circuit breaker device which is capable of detecting a short-circuit fault without necessarily involving the opening of said circuit breaker, since the opening operation may be subject to other parameters and/or assigned to other circuit breakers present in the network.
The objective of the present invention is to solve the technical problem illustrated above, by overcoming the limitations of the prior art, and in particular to carry out the rapid detection of short-circuit conditions in a network, with simultaneous calculation of the value of the power factor of the short-circuit (cos xcfx86) and of the phase of the voltage at the moment of the short-circuit.
These objectives are achieved by means of the invention which relates to a method for detecting short-circuit conditions in an electrical network comprising at least one voltage source connected to at least one load, characterized in that it comprises the following operational phases:
a) measuring the values of the instantaneous current i(t) flowing in the load and of its first derivative di(t)/dt;
b) performing N successive samplings of the values of the instantaneous current i(t);
c) estimating the effective value of the load R, L viewed by the said source;
d) generating a short-circuit detection signal C when the value of the estimated load is such as to cause the steady peak current to exceed an assigned threshold level Icc.
The invention relates, moreover, to a device for detecting short-circuit conditions in an electrical network comprising at least one voltage source connected to at least one load, the said device being characterized in that it comprises:
an analogue differentiator which receives as input a signal proportional to the instantaneous current i(t) and delivers as output a signal which represents the first derivative di(t)/dt thereof;
two analogue/digital converters whose inputs are connected to the said signal i(t) and to the output of the said differentiator; and
processing and detection means connected to the outputs of the said converters and able to output a signal representative of the short-circuit conditions C.
The invention relates, moreover, to a device for detecting short-circuit conditions in an electrical network comprising at least one voltage source connected to at least one load, the said device being characterized in that it comprises a digital/analogue converter which receives as input a signal proportional to the instantaneous current i(t), a digital differentiator (or filter) connected to the output of said converter, and processing and detection means, connected to the outputs of said converters and digital differentiator, said means being able to output a signal representative of the short-circuit conditions C.
According to the invention, the detection of short-circuit conditions is based on estimating the electrical characteristics of the load under short-circuit conditions, and more particularly its impedance. The peak value of the current is typically calculated from this, before the current actually attains the said peak value. If this value is greater than a desired threshold Icc, then the fault is detectd as a short-circuit.
Alternatively, more complex fault conditions can be defined, in which the threshold value Icc depends upon the value of cos xcfx86.
The method according to the invention makes it possible to achieve detection very rapidly, typically in a few milliseconds.
Advantageously, the method according to the invention makes it possible concurrently to determine other characteristics of the circuit at the moment of the fault, such as the value of the power factor of the short-circuit (cos xcfx86) and the phase of the voltage at the moment of the short-circuit.